It is known to introduce counter-vibrations into a structure to oppose and cancel external disturbance vibrations therein, to achieve net vibration reduction in the structure. This has been implemented by various control methods. See, e.g., U.S. Pat. No. 4,122,303 ("Improvements In And Relating To Active Sound Attenuation"); U.S. Pat. No. 4,153,815 ("Active Attenuation Of Recurring Sounds"); U.S. Pat. No. 4,417,098 ("Method Of Reducing The Adaptation Time In The Cancellation Of Repetitive Vibration"); U.S. Pat. No. 4,489,441 ("Method And Apparatus For Cancelling Vibration"); U.S. Pat. No. 4,490,841 ("Method And Apparatus For Cancelling Vibrations"); and U.S. Pat. No. 4,819,182 ("Method And Apparatus For Reducing Vibrations Of A Helicopter Fuselage").
If the external disturbance is sinusoidal, the solutions suggested by the prior art have generally attempted to synthesize a similar sinusoidal signal having the same frequency as the external vibration, which signal is subsequently adjusted in phase and amplitude by amounts that are adaptively estimated from the sensed residual vibration in the structure. The synthesized signal is then fed as a command to a vibratory force generator, the output of which is applied to oppose and cancel the sensed vibration in the structure. For such a scheme to be practical, it is necessary to synchronize the sinewave generator in frequency and/or time with the sensed vibration. In a typical implementation with an engine serving as the vibration source, an engine RPM signal or crankshaft position information is used to provide the needed frequency reference.
The iteratively-adaptive algorithms in the above patents are capable of controlling any type of periodic vibration, even those that are not sinusoidal. The vibration control system in the '182 patent, supra, is also aimed at a multi-sensor, multi-actuator vibration control problem. The complexity of such prior art solutions has required relatively-powerful digital computers for their implementation.
Another prior art solution, Val, Gregory and Gupta, "Design And Evaluation of a State-Feedback Vibration Controller", Journal of American Helicopter Society, 29:4 (July 1984), presents a continuous linear analog controller approach to the vibration cancellation problem based on modern state-feedback control theory. However, it is time-invariant, and therefore lacks frequency tracking capability.